The relationship between environmental factors, community composition and the sensitivity of pelagic phytoplankton to the antifouling agents zinc pyrithione (ZPT) and copper pyrithione (CPT) was studied using phytoplankton communities collected from March until August 2001 in Roskilde fjord, Denmark. Sensitivity to ZPT and CPT was measured as EC50 values obtained from dose-response curves of photosynthesis to ZPT and CPT. EC50 for ZPT and CPT varied between 2 and 60 nM and 4 and 25 nM, respectively. Changes in sensitivity throughout the season were related to changes in phytoplankton community composition and density, and to nutrient levels. It was found that the variation in sensitivity of ZPT and CPT was related to the abundance of the groups Cryptophyceae, Bacillariophycaea and Dinophyceae when they were dominating the community. Furthermore, the sensitivity to ZPT was increased at low concentrations of phosphate per cell (<0.2 nmol/cell). For CPT there was a negative correlation between toxicity and phosphate concentration in the water. Consequently, in aquatic environments where phytoplankton is phosphate limited the effect of ZPT and CPT may be enhanced.